This invention relates in general to loudspeakers which produce sound in response to an audio signal, and more particularly to a loudspeaker with an improved air cooling system.
Conventional loudspeakers typically employ a diaphragm which is vibrated by an electromechanical driver. The driver generally comprises a permanent magnet and a voice coil through which an electrical signal is passed from an audio amplifier. Changing voltage in the audio frequency range is applied to the terminals of the voice coil causing a corresponding changing current to flow through the windings of the voice coil. The interaction between the current passing through the voice coil and the magnetic field produced by the permanent magnet causes the voice coil to oscillate in accordance with the electrical signal. Since the voice coil is rigidly attached to the a diaphragm, oscillation of the voice coil causes a corresponding oscillation in the diaphragm to produce acoustical output.
A substantial portion of the impedance associated with electromechanical drivers is caused by the wire that forms the voice coil due to the wire""s DC resistance. Accordingly, most of the electrical power applied to the voice coil is converted into heat rather than sound. The ultimate power handling capacity of the voice coil, and thus the loudspeaker, is limited by the ability of the device to tolerate heat. Heat tolerance is generally determined by the lowest melting point of wire insulation and other components, as well as the heat capacity of the adhesive used to construct the voice coil.
The problems produced by heat generation are further compounded by temperature-induced resistance, commonly referred to as power compression. As the temperature of the voice coil increases, the DC resistance of the copper or aluminum conductors or wires used in the voice coil also increases, resulting in progressively decreasing efficiency. For example, a copper wire voice coil that has a DC resistance of eight ohms at 68xc2x0 C. will have a DC resistance of 16 ohms at 270xc2x0 C. At 270xc2x0 C., the voice coil will draw less power from the voltage applied to its terminals, and a substantial portion of the power that it does draw will be converted into heat. Consequently, the loudspeaker, which is a relatively inefficient transducer at room temperature, will be further reduced in efficiency at high voice coil temperatures. This power compression increases as the voltage applied to the voice coil increases, and can reach a point where a further increase in applied voltage results in virtually no increase in acoustical output, only a further increase in heat.
It is therefore desirable to provide a loudspeaker with a voice coil that can be cooled during operation. Reducing voice coil temperature will increase both the efficiency and power capacity of a loudspeaker; as well as its reliability and service life.
The prior art offers different solutions to voice coil cooling. By way of example, U.S. Pat. No. 4,757,547 issued to Danley on Jul. 12, 1988, discloses an air-cooled loudspeaker that has a voice coil positioned in an annular gap formed by pole pieces of a permanent magnet. The voice coil is cooled by directing pressurized air through the gap and over the voice coil. Typically, the clearances between the voice coil and the boundaries of the gap are quite small, usually under 0.020 inch. In order to adequately cool the voice coil, air must be forced through these clearances at a relatively high air flow rate and pressure which, consequently, can cause undesirable noise and distortion in the loudspeaker.
U.S. Pat. No. 5,042,072 issued to Button on Aug. 20, 1991, discloses a self-cooled loudspeaker that has a voice coil positioned in an annular gap between a permanent magnet and a pole piece. Axially extending air channels are formed at particular locations around the circumference of the pole piece to cool portions of the voice coil. Although this structure does not require forcing pressurized air through a relatively small gap, there is a reduction of magnetic flux at the axial air passages since portions of the pole piece have been cut away.
U.S. Pat. No. 5,357,586 issued to Nordschow et al. on Oct. 18, 1994, discloses an air-cooled loudspeaker system having aerodynamically-shaped passages that primarily cool the magnetic structure through induced airflow from vibratory movement of a speaker cone. The only direct cooling of the voice coil results from air flowing in the narrow clearances between the voice coil and the boundaries of the magnetic gap. Because of the relatively low air pressure created by the induced airflow, relatively little air will actually flow over the voice coil to cool it.
According to the invention, a loudspeaker comprises a speaker frame and a diaphragm connected to the speaker frame for reciprocal movement relative thereto. A generally tubular former is connected to the diaphragm, and a voice coil is connected to the former at a location spaced from the diaphragm. The former is constructed of a thermally conductive material for conducting heat away from the voice coil. A permanent magnet has a central opening and a pole piece has a pole vent opening that is coincident with the central opening. The voice coil is located in a space formed between the permanent magnet and the pole piece. An airflow director is positioned at least partially in the former, with a first gap being formed between the airflow director and an inner surface of the former and a second gap being formed between the airflow director and the pole piece. The first and second gaps are in fluid communication with each other and the pole vent opening such that movement of the diaphragm causes airflow through the first and second gaps and the pole vent opening.
With this construction, heat generated in the coil during operation of the loudspeaker is transferred to the former through conduction, and heat present in the former is transferred through the first and second gaps and the pole vent opening through convection to thereby cool the loudspeaker.